Two former Parramatta Eels players are accused of harbouring semi-automatic weapons and possessing more than half-a-million dollars in cash after dramatic arrests in Sydney's Centennial Park yesterday.

A man is in hospital with serious injuries after crashing off a home-made billycart as he sped down a reservoir spillway near Williamstown in South Australia.

Police say three men aged between 32 and 44 were taking turns on the billycart at the South Para Reservoir just after 5pm on Friday when the 44-year-old man came off at high speed on the steep concrete slope.

He suffered serious but non-life threatening head injuries and abrasions and was airlifted to Royal Adelaide Hospital.

Police say the three men will be fined more than $300 each for trespassing and the seized billycart will be destroyed.